what is the best city in Europe to fly to from New York City in order to get to Split airport
Google "NYC to Split flights" to see your options. I assume it's not "best city" you're after, but the most convenient itinerary combined with the best price (and maybe other criteria). That depends on the dates of travel, departing airport, and airline (including codeshare if the main airline codeshares with another airline). Airfares change all the time so there's no clear answer to your question.
Thanks. I'll follow your suggestion and see what happens.